1. Field of the Invention
The present invention pertains to the use of dialkyl dicarbonates combined with decomposition catalysts as blowing agents for polymeric compositions which rapidly progress from a fluid to a rigid physical form over a time-temperature profile which is inadequate to provide substantial thermal decomposition of the dicarbonates.
2. Background of the Invention
Blowing agents are used in combination with polymers to produce polymeric foams. Polymeric foams having broad application include, for example, urethane foams, polyester foams, and foams comprising polyvinyl chloride. Blowing agents for polymers fall into two categories, chemical blowing agents and physical blowing agents. Commercially used chemical blowing agents are dominated by azodicarbonamide and physical blowing agents by chlorofluorocarbons and methylene dichloride; both of these compound types have toxicological and ecological problems associated with them.
It would be desirable to have a blowing agent for polymers which comprises carbon dioxide, since this blowing agent is non-toxic and environmentally acceptable. Many organic compounds when heated evolve carbon dioxide; unfortunately, the temperature at which this occurs precludes their use in many polymeric foaming applications. However, there are particular organic compounds which can be made to decompose, releasing carbon dioxide, at lower temperatures when used in combination with a suitable decomposition catalyst.
U.S. Pat. No. 3,573,232 to Kloeker et al., issued Mar. 30, 1971, describes a process for the production of polyester foamed materials. A copolymerizable mixture of unsaturated polyesters and monomeric vinyl compound employs as a foaming agent a carbonic acid ester anhydride. A polyvalent metal compound is used as a catalyst to cause decomposition of the anhydride without the application of a substantial amount of heat.
U.S. Pat. No. 3,573,233 to Krefeid et al., issued Mar. 30, 1971, discloses a process of producing foamed materials from a copolymerizable mixture of unsaturated polyesters and a monomeric polymerizable vinyl compound employing as a foaming agent a carbonic acid ester anhydride. Particular amine compounds consisting of a primary or an acyclic or cyclic secondary or an acyclic or cyclic tertiary amine with alkyl, alkenyl, or alkenyl radicals or its quaternary ammonium base or a primary or secondary N-monoaryl amine are used as catalysts to cause decomposition of the anhydride without the application of a substantial amount of heat.
U.S. Pat. No. 4,070,310 to Schneider et al., issued Jan. 24, 1978, describes the use of a process for the production of polyurethane foams preferably having a compact surface, in which process a mixture of polyisocyanates and compounds which liberate carbon dioxide under the catalytic influence of basic compounds is reacted with organic compounds containing hydrogen atoms which are reactive with isocyanate groups in a closed mould. The organic compounds containing reactive hydrogen atoms are mixed with basic compounds and/or have basic compounds chemically built into them; the reaction can be carried out in the presence of auxiliary agents and additives which are known in the chemistry of polyurethane foams. The reaction which generates carbon dioxide is carried out in the presence of water and/or organic blowing agent. The presence of the carbon dioxide increases the time period the foam is in a low viscosity state, thus improving the flow properties of the foamable mixture. The patent indicates in the paragraph bridging columns 2 and 3 that the compounds releasing carbon dioxide are not used as blowing agents and would be unsuitable as sole blowing agents.
The organic compounds which split off carbon dioxide under the catalytic influence of basic compounds include pyrocarbonic acid dimethylester, pyrocarbonic acid diethyl ester, pyrocarbonic acid dibutyl ester, pyrocarbonic acid dioctadecyl ester, acetic acid - carbonic acid ethyl ester anhydride, propionic acid - carbonic acid - ethyl ester anhydride, sebacic acid - bis (carbonic acid methyl ester) - anhydride, adipic acid bis - (carbonic acid methyl ester) - anhydride, crotonic acid carbonic acid - methyl ester - anhydride and the like. The use of carbonic acid ester anhydrides or mixed anhydrides and basic compounds to produce carbon dioxide is said to be unsuitable as the sole blowing agent for the foam, particularly for molded foam products.
Carbonic acid esters and ester anhydrides are used in combination with any inorganic or organic compounds which are basic in reaction, to produce the carbon dioxide. Examples of basic compounds used as decomposition catalysts include alkali metal hydroxide or alcoholates such as sodium hydroxide, potassium hydroxide, sodium ethylate and potassium methylate and salts which are basic in reaction. Preferred basic compounds are tertiary amines. Examples of such amines include triethylamine, dimethylbenzylamine, permethylated diethylene triamine and triethylenediamine.
U.S. Pat. No. 4,110,273 to Cohnen et al., issued Aug. 29, 1978 discloses a blowing agent composition consisting of 1,4 butanediol-bis-(carbonic acid ester-benzoic acid anhydride) and silicon dioxide. The silicon dioxide is credited with reducing the decomposition temperature of the anhydride while increasing the yield of gas substantially. This blowing agent is used for foaming thermoplastics such as polycarbonates, polyesters, polyamides, and mixtures of polyphenylene ethers and polystyrene at temperatures ranging from about 160.degree. C. to about 300.degree. C.
U.S. Pat. No. 4,297,442 to Blahak, issued Oct. 27, 1981, describes cellular elastomeric foams produced by foaming a polyurethane or a polyurethane prepolymer crosslinked with a polyamine chain lengthening agent in the presence of an organic expanding agent containing at least in part a gas forming component which reacts with the polyamine chain lengthening agent or with the products of reaction thereof with polyisocyanates and split-off gas. The organic expanding agents include organic solvents and components that decompose at temperatures above room temperature (e.g. 55.degree. C.), splitting off gases on decomposition. Examples of expanding agents which react with amine chain lengtheners or with the reaction product of the chain lengthener with polyisocyanates include dicarbonic-acid dialkylesters, alkyl carbaminates and Leusche anhydrides.
French patent application, Publication No. 2,220,564, by Societe Nationale des Poudres et Explosifs, filed Mar. 6, 1973, describes organic dicarbonates useful as blowing agents for plastic materials. The application pertains to the use of dicarbonates comprising a t-butyl group or two isopropyl groups, since these dicarbonates are said to be transformed almost completely into gas on thermal decomposition, producing an increased yield of carbon dioxide, whereas other dicarbonates typically are not. In addition, the presence of the t-butyl group is shown to provide a pronounced reduction in the decomposition temperature of the dicarbonate, with maximum gas production taking place at about 140.degree. C. The subject matter of this publication provides background information helpful in understanding the present invention and is hereby incorporated by reference.
The patent to Werner and Mertz, DE 2,053,399, apparently describes the use of dimethyl and diethyl dicarbonates in conjunction with water to produce foams from unsaturated polyester resins and other resins. However, the method relies on the thermal hydrolysis of the dicarbonate to produce the gas necessary for foaming and apparently does not employ a catalyst to promote the hydrolysis. The data presented by J. Drozd, J. Novak and S. Wicar in "Analysis of the Rate of Degradation of Diethyl Dicarbonate by Gas Chromatography", J. Food Sci., Vol. 40, 1109 (1975), would indicate that the rate of hydrolysis of diethyl dicarbonate at ambient temperatures would be inadequate for foaming but that at elevated temperatures a more acceptable rate is obtained. The system of the present invention differs from Werner and Mertz in providing a blowing agent system, suited to the production of various polymer foams, which exploits the rapid hydrolysis of dialkyl dicarbonates at ambient temperatures in the presence of specific tertiary amine catalysts.
The majority of sources cited above use gas forming components which initiate the gas generation at temperatures only above about 90.degree. C., with one example at about 55.degree. C. However, there are numerous applications for which a foam generating component which is active at lower temperatures such as ambient (room) temperature is desired.
In addition to having a gas generating agent which functions at room temperatures, (over a range from about 10.degree. C. to about 45.degree. C.) it is important to have the time period required to create the foam be balanced with the polymer gelation time. The foam creation time period for the decomposition of pyrocarbonic acid esters and carbonic acid ester anhydrides at the processing (reaction) temperature of the polymeric composition is of critical importance in the production of foamed polymeric systems which rely on the decomposition process as a major source of blowing agent. Accordingly, it is an object of the present invention to provide improved methods of controlling the rate and amounts of gases evolved from blowing agents comprising dialkyl dicarbonates.